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FLAT-PLATE COLLECTORS Solar Energy I Physics 471 2004-1 Instructor : Prof. Dr. AHMET ECEVIT Presented by: YASIN G

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Title: FLAT-PLATE COLLECTORS Solar Energy I Physics 471 2004-1 Instructor : Prof. Dr. AHMET ECEVIT Presented by: YASIN G


1
FLAT-PLATE COLLECTORS Solar Energy I Physics
471 2004-1 Instructor Prof. Dr. AHMET ECEVIT
Presented by YASIN GÜNERI
2
TABLE OF CONTENT



PAGES
  • 1)INTRODUCTION

    3
  • 2)FLAT-PLATE COLLECTORS

    5
  • A. ABSORBER PLATE FLOW PASSAGES

    9
  • B.COVER PLATES

    12
  • C.ENCLOSURE / INSULATION

    15
  • 3) PROPER ORIENTATION AND ANGLE of SOLAR
    COLLECTOR 17
  • A. FLAT-PLATR COLLECTORS FACING SOUTH AT
    FIXED TILT 18
  • B. ONE-AXIS TRACKING FLAT-PLATE COLLECTORS
    WHIT AXIS ORIENTED 19
  • NORTH-SOUTH

  • C. TWO-AXIS TRACKING FLAT-PLATE COLLECTORS

    20
  • 4) COLLECTOR PERFORMANCE

    21
  • A. ABSORBED RADIATION

    25
  • B. COLLECTOR HEAT REMOVAL FACTOR

    26
  • C. OVERALL HEAT LOSS COEFFICIENT

    27
  • 5) COLLECTOR EFFICIENCY

    29
  • 6) APPLICATIONS

    32
  • A. DOMESTIC APPLICATIONS

    33
  • B. COMMERCIAL APPLICATTIONS

    35


  • 7) CONCLUSION

    37

3
1. INTRODUCTION
  • Solar collectors are heat exchangers that use
    solar radiation to heat a working fluid, usually
    liquid or air. They can be classified in three
    groups
  • - Flat-plate collectors,
  • - Evacuated-tube collectors
  • - Focusing collectors.

4
  • In flat-plate collectors there is no optical
    concentration of sunlight and they are generally
    stationary . In addition to this their outlet
    temperature capability is below 100 C
  • However to reach higher temparatures
    evacuated-tube collectors and focusing collectors
    are used.






    In evacuated-tube collectors they use
    vacuun to reduce heat lost and to protect the
    absorber coating from deteration.By this way they
    can reach temperatures up to 140 C and they
    can collect both direct and diffuse solar
    radiation
  • And focusing collectors, they are not stable and
    they follow the sun to get direct radiation
    theycan not utilize diffuse radiation. And they
    are also capable of producing high temperatures
    1.

5
2. FLAT-PLATE COLLECTORS
  • A flat plate collector is basicly a black
    surface that is placed at a convenient path of
    the sun.And a typical flat plate collector is a
    metal box with a glass or plastic cover (called
    glazing) on top and a dark-colored absorber plate
    on the bottom. The sides and bottom of the
    collector are usually insulated to minimize heat
    loss.2
  • Figure 2.1 gives examples of flat-plate
    collectors
  • Figure 2.1 Flat-plate collectors3.


6
  • Components of a typical flat plate collector
  • Absorber plate
  • It is usually made of copper,steel or
    plastic.The surface is covered with a flat black
    metarial of high absorptance.If copper or steel
    is used it is possible to apply a selective
    coating that maximizes the absorptance of solar
    energy and minimizes the radiation emitted by
    plate.
  • Flow passages
  • The flow passages conduct the working fluid
    through the collector. If the working fluid is a
    liquid , the flow passage is usually a tube that
    is attached to or is a part of absorber plate.
    If the working fluid is air , the flow passage
    should be below the absorber plate to minimize
    heat lssos.

7
  • Cover plate
  • To reduce convective and radiative heat
    losses from the absorber , one or two transparent
    covers are generally placed above the absorber
    plate.They usually be made from glass or plastic.
  • Insulation
  • These are some metarials such as fiberglass
    and they are placed at the back and sides of the
    collector to rduce heat losses.
  • Enclosure
  • A box that the collector is enclosed in holds
    the componrnts together, protect them from
    weather, facilitates installation of the
    collector on a roof or appropriate frame 1.

8
  • Here in figure 2.2 we can see components of flat
    plate
  • collectors.
  • Figure 2.2 Cross section of a basic flat-plate
    solar collector 4.

9
A. Absorber plate Flow passages
  • Copper,which has high conductivity and is
    corrosion-resistant, is the material for absorber
    plates, but because copper is expensive, steel is
    also widely used. For a copper plate 0.05 cm
    thick with 1.25-cm tubes spaced 15 cm apart in
    good thermal contact with the copper, the fin
    efficiency is better than 97 percent.
  • The surface of the absorber plate determines
    how much of the incident solar radiation is
    absorbed and how much is emitted at a given
    temperature. Flat black paint which is widely
    used as a coating has an absorptance of about 95
    percent for incident shortwave solar radiation.
    It is durable and easy to apply 1.

10
  • Here a table about matters that absorber
    plate may be made from
  • Table 2.1 Characteristics of absorptive coatings
    1.

Material Absorptance (?) Emittance (?) Break down temparature (C) Comments
Black silicon paint 0.86-0.94 0.83-0.89 350 Slicone binder
Black silicon paint 0.9 0.5 Stable at high temperature
Black copper over copper 0.85-0.9 0.08-0.12 450 Patinates with moisture
Black chorome over nickel 0.92-0.94 0.07-0.12 450 Stable at high temperatures
11
  • Here in figure 2.3 we can see absorber plate
    and flow
  • passages
  • Figure 2.3 Cross section of a absorber
    plateflow passages of a flat
  • plate collector 4.

12
B. Cover plates
  • A cover plate for a collector should have a
    high transmittance for solar radiation and should
    not detoriate with time. The material most
    commonly used is glass. A 0.32-cm thick sheet of
    window glass ( iron content, 0.12 percent )
    transmits 85 percent of solar energy at normal
    incidence. And all glass is practically opaque to
    long-wavelength radiation emitted by the absorber
    plate.
  • Some plastic materials can be used for
    collector glazing.They are cheaper and lighter
    than glass and, because they can be used in very
    thin sheets, they often have higher
    transmittance. However, they are not as durable
    as glass and they often degrade with exposure to
    ultraviolet radiation or high temperatures 1.

13
  • Here a table about matters that cover plate may
    be made from

Test Polyvinly floride Polyethylene terephthatalet or polyster Polycarbonate Fiberglass rein forced plastics
Solar Transmission, 92-94 85 82-89 77-90
Maximu operating temperature C 110 100 120-135 95
Thermal Expansion Coefficient 43 27 68 32-40
Thickness, mm 0.1 0.025 3.2 1.0
Length of life, years In 5 years 95 retains 4 7-20
14
  • Here in figure 2.4 we can see cover part.
  • Figure 2.4 Cross section of a cover part of a
    flat-plate collector 4.

15
C. Enclosure / Insulation
  • The collector enclosure is usually made from
    steel, aliminium or fiber glass.And order to
    prevent heat from escaping through the back of
    the collector,a layer of insulation is placed
    behind the absorber plate 1.
  • Table 2.3 Characteristics of insulation
    materials 1.

Material Density Kg/m3 Thermal conductivity at 95 C (W/mK) Temperature limits C
Fiber glass with organic binder 11 0.059 175
16 0.050 175
24 0.045 175
48 0.43 175
16
  • Here in figure 2.5 we can see insulation part.
  • Figure 2.5 Cross Section of an Insulation Part of
    a Flat-Plate Collector 4.

17
3. PROPER ORIENTATION and ANGLE of SOLAR
COLLECTOR
  • Flat plate collectorts are divided in three main
  • groups according to how they are oriented
  • Flat-plate collectors facing south at fixed tilt
  • One-axis tracking flat-plate collectors with axis
    oriented north-south
  • Two-axis tracking flat-plate collectors

18
A. Flat-plate collectors facing south at fixed
tilt
  • To optimize performance in the winter, the
    collector can be tilted 15 greater than the
    latitude to optimize performance in the summer,
    the collector can be tilted 15 less than the
    latitude 5. Figure 3.1 show how the collector
    is tilted.
  • Figure 3.1 Flat-plate collector at fixed tilt 5.

19
B. One-axis tracking flat-plate collectors with
axis oriented north-south
  • These trackers pivot on their single axis to
    track the sun, facing east in the morning and
    west in the afternoon as shown in figure 3.2.
  • Figure 3.2 Flat-plate collector one axis
    tracking5.

20
C. Two-axis tracking flat-plate collectors
  • Tracking the sun in both azimuth and
    elevation, these collectors keep the sun's rays
    normal to the collector surface as shown in
    figure 3.3.
  • Figure 3.3 Flat-plate collector with two axis
    tracking5.

21
4.COLLECTOR PERFORMANCE
  • The thermal performance of a collector can be
    calculated from a first-law energy balance.
    according to the first law of thermodynamics, for
    a simple flat-plate collector an instantaneous
    steady-state energy balance is1
  • Useful energy energy absorbed heat loss to
  • gain (Qu) by the collector
    surroundings

22
  • And,
  • Absorbed energy AC FR S
  • Lost energy AC FR UL (Ti-Ta)
  • where
  • AC Collector area, m2
  • FR Heat removal factor, unitless
  • S Absorbed solar radiation, J/m2
  • UL Heat transfer loss coefficient, J/m2 C
  • Ti The mean absorber plate temperature, C
  • Ta The ambient temperature, C.

23
  • So
  • Equation 4.1 Useful gain enerrgy equation6.

QU AC FR S - AC FR UL (Ti-Ta)
24
  • Equation 4.1 is an extremely useful equation and
  • applies to essentialy all flat-plate collectors.
  • And to improve theperformance of solar collector
    it is
  • necesssary either to reduce the overall energy
    loss
  • coefficient or reduce area from which energy is
    lost.
  • That is the maximum possible useful energy gain
    (heat
  • transfer) in a solar collector occurs when the
    whole
  • collector is at the inlet fluid temperature heat
    losses to
  • the surroundings are then at a minimum 1,6.

25
A. Absorbed radiation (S)
  • In equation 4.1 S is absorbed radiation and it is
    equal to
  • Equation 4.2 Absorbed solar radiation6.
  • In equation 4.2
    are
  • the view factors from the collector to the sky
    and
  • from the collector to the ground, respectively.
  • The subscripts b,d, and g represent beam,
  • diffuse, and ground , respectively.
    is
  • transmittance and absorptance product.Rb is the
  • ratio of beam radiation on the tilted surface to
    that on a
  • horizantal surface at any time6.

26
B. Collector heat removal factor (FR)
  • In equation 4.1 FR is collector heat removal
    factor a quantity that relates the
  • actual useful energy gain of a collector to the
    useful gain if the whole collector
  • surfaces were at the fluid inlet temperature6.
    And it is given by equation 4.3.
  • Equation 4.3 the collector heat removal factor
    FR 6.
  • Where
  • m Fluid mass flow rate, kg/s
  • Cp Fluid specific heat, J/kg C
  • The quantitiy FR is equavialent to the
    effectiveness of a
  • conventional heat exchange, which is defined as
    the ratio of the actual
  • heat transfer to the maximum possible heat
    transfer. The maximum
  • possible useful energy gain (heat transfer) in a
    solar collector occurs
  • when the all whole collector is at the inlet
    fluid temperature heat
  • losses to the surroudings are than at a minimum
    6.

27
C. Overall heat loss coefficient (UL)
  • In equation 4.1 UL is the collector overall loss
  • coefficient and it is equal to the sum of the
    top,
  • bottom,and edge loss coefficients 6
  • Equation 4.4 Overall loss coefficient UL 6.

ULUtopUbottomUedge,W/m²K
28
  • Energy diagram of typical flat flate
    collector is shown in figure 5.1. 92 of the
    total sunshine reaches to the copper absorber. 8
    of the total sunshine is reflected from glass. 5
    of the sunshine is emitted from the panel, 12 is
    lost through convection and conduction.
  • Figure 5.1 Energy diagram for typical flat plate
    collector 3

29
5. COLLECTOR EFFICIENCY
  • The basic method of measuring collector
    performance is to expose the operating collector
    to solar radiation and measure the fluid inlet
    and outlet temperatures and the fluid flow
    rate.The useful gain is 6
  • Equation 5.1 Energy gained
    by liquid6.
  • Where
  • m Fluid mass flow rate, kg/s
  • Cp Fluid specific heat, J/kgC

30
  • The equation 5.1 which describes the thermal
    performance of a collector operating under steady
    conditions, can be rewritten 6
  • Equation 5.2 Useful gain enerrgy
    equation6.
  • Where is a transmittance-absorptance
    product that is weighted according to the
    proportions of beam, diffuse, and ground
    reflected radiation on the collector 6.

31
  • And finally instantaneous efficiency can be
    defined as 6
  • That is

32
6) APPLICATIONS
  • Flat plate collectors are used for both
  • A) Domestic applications
  • B) Commercial applications

33
A) Domestic applications
  • Flate plate collectors mainly used in
    residential buildings where
  • the demand for hot water has a large impact on
    energy bills. This
  • generally means a situation with a large family,
    or a situation in
  • which the hot water demand is excessive due to
    frequent laundry
  • washing 2.
  • For instance, a family of 4 members consumes
    on an average
  • 100 litre of hot water a day at 60 C. Hot water
    of 100 litre
  • capacity at 60 C approximate can be delivered by
    a single
  • collector system of 2 m² area. The solar water
    heating systems are
  • generally provided with auxiliary backup in the
    insulated hot
  • storage tank for the rainy and heavily overcast
    cloudy days 7.

34
  • Here we can see solar flat-plate collectors
    used for heating buildings.
  • Figure 6.1 Flat plate collectors used for heating
    buildings 8.

35
B) Commercial applications
Commercial applications include laundromats,
car washes, military laundry facilities and
eating establishments. Solar water heating
systems are most likely to be cost effective for
facilities with water heating systems that are
expensive to operate, or with operations such as
laundries or kitchens that require large
quantities of hot water. And unglazed
liquid collectors are commonly used to heat water
for swimming pools. Because these collectors need
not withstand high temperatures, they can use
lessexpensive materials such as plastic or
rubber. They also do not require freeze-proofing
because swimming pools are generally used only in
warm weather or can be drained easily during cold
weather 2.

36
  • Here we can see solar flat-plate collectors
    used for heating swimming pools.
  • Figure 6.2 Flat-plate collectors used for heating
    swimming pools 9.

37
7) CONCLUSION
  • Flat-plate collectors which are used for water
    heating,
  • are long lasting, and also in long term they are
    cheaper
  • than other water heating systems.However,they
    requires
  • large areas if high energy output is a
    requirement.
  • Than solar energy is free if we do not include
    the initial
  • cost for installation and the maintenance.
  • Finally bessides these we should remember by
    using
  • solar energy we can protect nature.

38
REFERENCES
  • 1 Jan F. Kreider, Charles J. Hoogendoorn,
  • Frank Kreith Solar Design Hemisphere
  • Publishing Corporation, (1989), pp. 44-55.
  • 2 http//www.flasolar.com
  • 3 http//www.solarnetrix.com
  • 4 http//www.solstice.crest.org
  • 5 http//www.rredc.nrel.gov
  • 6 Duffie, J. A. and Beckman, W. A. , 1991.
    Solar
  • Engineering of Thermal Processes , John Wiley and
  • Sons Inc., New York, pp.250-290 .
  • 7 http//www.iredaltd.com
  • 8 http//www.ips-solar.com
  • 9 http//www.northeastpoolstore.com
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